A. Field of the Invention
The present invention pertains generally to optical communications, and more particularly to a variable semiconductor all-optical buffer where electromagnetically induced transparency is used to slow light.
B. Publications Incorporated By Reference
The following publications are hereby incorporated by reference:
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9. K. L. Vodopyanov, G. B. Serapiglia, C. Sirtori, and J. Faist, “Electromagnetically induced transparency in a three-subband semiconductor quantum well,” QELS, pp. 258, May 1999.
10. C. C. Phillips, E. Paspalakis, G. B. Serapiglia, C. Sirtori, and K. L. Vodopyanov, “Observation of electromagnetically induced transparency and measurements of subband dynamics in a semiconductor quantum well,” Physica E 7, pp. 166–173 (2000).
11. K. D. Choquette, K. M. Geib, C. I. H. Ashby, R. D. Twesten, O. Blum, H. Q. Hou, D. F. Follstaedt, B. E. Hammons, D. Mathes, and R. Hull, “Advances in selective wet oxidation of AlGaAs Alloys”, IEEE J. of Selected Topics Quantum Electron., vol. 3, pp. 916–926 (1997).
12. R. Langenhorst, M. Eiselt, W. Pieper, G. Grosskopf, R. Ludwig, L. Kuller, E. Dietrich, and H. G. Weber, “Fiber loop optical buffer,” J. Lightwave Technol., 14, 3, pp. 324–335, March 1996.
13. J. D. Moores, K. L. Hall, S. M. LePage, K. A. Rauschenbach, W. S. Wong, H. A. Haus, and E. P. Ippen, “20-GHz optical storage loop/laser using amplitude modulation, filtering, and artificial fast saturable absorption,” IEEE Photon. Technol. Lett., vol. 7, pp. 1096–1098, September 1995.
14. K. L. Hall, J. D. Moores, K. A. Rauschenbach, W. S. Wong, E. P. Ippen, and H. A. Haus, “All-optical storage of a 1.25 kb packet at 10 Gb/s,” IEEE Photon. Technol. Lett., 7, p. 1093 (1995).
15. K. L. Hall, “40-Gbit/s optical packet buffering,” Proc. Conf. OFC, ThD3, pp. 251–252 (1997).
16. J. P. Marangos, “Electromagnetically induced transparency,” J. Modern Optics 45, 471 (1998).
17. D. F. Phillips, A. Fleischhauer, A. Mair, R. L. Walsworth, and M. D. Lukin, “Storage of Light in Atomic Vapor,” Phys. Rev. Lett. 86, 783 (2001).
18. A. V. Turukhin, V. S. Sudarshanam, M. S. Shahriar, J. A. Musser, B. S. Ham, and P. R. Hemmer, “Observation of Ultraslow and Stored Light Pulses in a Solid,” Phys. Rev. Lett. 88, 023602 (2002).
19. J. B. Khurgin, “Light slowing down in Moire fiber gratings and its implications for nonlinear optics,” Phys. Rev. A 62, 013821 (2000).
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21. P. Borri, W. Langbein, S. Schneider, U. Woggon, R. L. Sellin, D. Ouyang, and D. Bimberg, “Ultralong Dephasing Time in InGaAs Quantum Dots,” Phys. Rev. Lett. 87, 157401(2001).
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23. J. Mori et al., “1.3–1.5 μm wavelength quantum dots self-formed in GaAs/InAs superlattices grown on InP(411) substrates,” 2001 International Conference on Indium Phosphide and Related Materials, WP-63.
24. J. R. Guest, “Measurement of optical absorption by a single quantum dot exciton,” Phys. Rev. B, vol. 65, 241310R, 2002.
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C. Discussion of the Background Art
An optical fiber delay line, also known as a digital “optical buffer”, is one of the most important components for all-optical communications and optical-signal processing systems. Such a buffer must be able to store the data packets for a substantial period of time and must be able to release the data within an acceptable delay when the switch is clear for routing.
The major components of a single fiber optical buffer consists of a fiber loop, an optical isolator, 3-dB couplers, and several semiconductor laser amplifiers for the gating, interconnection, and loss compensation. All-optical storage of a 1.76 kb packet of 20 Gb/s pulsed and noise-generated data has been demonstrated using amplitude modulation, filtering, and artificial fast saturable absorption. In these experiments, a pump laser for pumping an erbium-doped fiber connected to a 12 m single-mode fiber is used with a LiNbO3 modulator. Loading and unloading of 40 Gb/s data packets have also been demonstrated using a fiber loop optical buffer using a similar setup.
The fundamental difficulty facing the design of an optical buffer is that variable-length buffers must be implemented with delay lines. However, by their nature, fiber loop optical delay lines are of fixed length. Once a packet has entered the delay line, it can only emerge at a fixed duration later. It is impossible to remove the packet from the delay line before that time interval. Therefore, since the delay is for a fixed amount of time, this type of buffer has very limited applications.
However, fiber delay lines are not the only manner in which light can be slowed. There have been major breakthroughs in achieving “slow light” using the principles developed under electromagnetically induced transparency (EIT). Experimentally, research groups have independently “slowed down” light propagation in two different materials, a gas of cold sodium atoms and a glass cell containing rubidium, which was heated up to create rubidium vapor. Slow-down factors as high as seven orders of magnitude (down to 17 m/s) have been reported in both atomic vapor cells and Pr doped Y2SiO5 crystals. The slow light principle is based on creating destructive interference between two optical transitions in electronic states by means of an optical pump field, modifying the dispersion spectrum experienced by the optical signal. Other mechanisms such as a Moire grating have also been proposed to modify the dispersive characteristics.
A large group velocity reduction can be achieved in a medium without loss and the minimum dispersion by looking at the relationship of vg(ω) and χ(ω), the susceptibility of the medium. The real and imaginary part of the susceptibility χ′(ω) and χ″(ω) are related by the Kramers-Kronig relations. The refractive index and the group velocity can then be expressed in terms of the real and imaginary part of the complex susceptibility. The group velocity can be expressed as
      v    g    =      c                  (                  n          +                      ω            ⁢                                                  ⁢                                          ∂                n                                            ∂                ω                                                    )                    ω        =                  ω          0                    
The dispersion parameter D (ps/nm-km) can be expressed as
  D  =            -                        2          ⁢                                          ⁢          π                          λ          2                      ⁢                  (                              2            ⁢                                                  ⁢                                          ⅆ                n                                            ⅆ                ω                                              +                      ω            ⁢                                                  ⁢                                                            ⅆ                  2                                ⁢                n                                            ⅆ                                  ω                  2                                                                    )                    ω        =                  ω          0                    
For a large velocity reduction, a large and positive dn(ω)/dω is necessary; whereas for a small D, a small and negative d2n(ω)/dω2 is desirable. For a small loss, χ″(ω) must be minimized.
A typical 2-level transition does not satisfy these criteria. However, these criteria can be met for a three level system, |1>, |2> and |3>. FIG. 1A shows a three-level (1, 2, 3) system with the coupling laser in resonance with states 2 and 3, resulting in a set of dressed states on the right. There are three basic energy level schemes for a three-level system interacting with two near-resonance electromagnetic fields, a ladder or cascade system (E1<E2<E3), a Λ scheme (E1<E3<E2), and a V-scheme (E2<E1 and E3), where, as can be seen in FIG. 1A, |1> to |2>, and |2> to |3> transitions are dipole allowed, while |1> to |3> is generally dipole forbidden (metastable). By optically pumping the system with an energy corresponding to E3−E2, we induce a coherence interaction between |2> and |3> and effectively split energy state |2> into two coherently coupled dressed states of |2d> and |3d>. More discussions on the subject of the electromagnetically induced transparency (EIT) effect and quantum coherence can be found in the literature. As the two new states are coherent, the real and imaginary susceptibility spectra for the dressed states can be represented by FIG. 1B. It can be seen that, for a photon energy in between the two transitions, the first derivative of n(ω) can be very large and positive. By carefully designing the linewidth of the two transitions, the absorption and the second derivative and, effectively, D can all be minimized. As the coherence of the two new states is provided by the optical pump beam, a controlled velocity reduction factor (optical memory size) may be obtained by tailoring the intensity or detuning of this pump beam.
In semiconductor materials systems, EIT has been observed at T=30 K using three levels in the quantized conduction band of an n-type doped quantum-well structure. In particular, it was found that the absorption of the 1 to 2 state transition in a 1-2-3 ladder configuration is significantly reduced when the pump (control) field is tuned to half energy of the 1 to 3 level transition energy with a simultaneous splitting of the absorption peak on both sides of the peak absorption wavelength, as expected from the imaginary part of the susceptibility (dashed curve) in FIG. 1B. As a result of the control field, the originally absorbing medium becomes transparent at the center frequency, yet the group velocity will decrease significantly as expected from the real part of the susceptibility (solid curve) in FIG. 1B.
As expected from a semiconductor quantum well (QW) structure, the density of states is continuous and the resulting EIT effect is small and observed only at low temperatures. The development of quantum dots (QD) and quantum wires for optical emitters is a topic of intense research due to the theoretical promise of ultra-low threshold currents and temperature independent emission characteristics of such structures. However, technical realization of the full potential of such structures is still seriously hampered by the variability of the currently available material that is based on the self-organized growth of such low-dimensional structures. The best laser results currently are obtained from self-assembled InAs QDs on a GaAs substrate. The effect of strain (lattice mismatch) appears to be a major cause for the improved optical quality.
Therefore, there is a need to for a technique to increase the coherent interaction of the states and obtain a large slow-down factor at room temperature. There is also a need for a medium that can slow down the group velocity of optical transmission with a controlled slow-down factor such that the medium is effectively an optical memory via true time delay. By controlling this group velocity reduction factor, the memory length can be adjusted to the desired values with minimum pulse dispersion or optical loss.
A further need exists for an optical buffer on compound semiconductor using photonic bandgap engineered quantum-dot (QD) devices. By using QD in photonic crystals, much sharper energy levels on semiconductors can be achieved, which is required for attaining EIT. This will result in group velocity reduction and thus switchable optical memory in such samples.
There have been a lot of research efforts on quantum dots (QD) and QD devices. To date, however, none has yielded optical devices with expected properties. This is primarily due to processing difficulty, which typically creates either highly nonuniform dots or dots with many surface defects. The nonuniformity introduces significant inhomogeneous broadening, which reduces the effectiveness of energy quantization. However, the defects lead to recombination centers, making the material inferior for optical applications.
There are many major advantages for all-optical buffers on compound semiconductor, rather than Si or optical fiber. Optical loss is a major trade-off with a large group velocity reduction and low dispersion. The use of compound semiconductors enables distributed integration of semiconductor optical amplifiers and buffers. Further integration with optical waveguiding structure, a control laser and optical coupler are all advantageous.